Computer-based systems software

For input, storage, manipulation, and output within computer-based systems, a representation of the chemical substance must be selected. The selection of a particular representation scheme for an information system is based on the size of the files to which it applies, the functions to be performed, the available hardware and software, and the desired balance between... 

Most large networked systems embody human beings as, in effect, system components , alongside the hardware and software components, though as indicated above such humans may become actively involved only when things go wrong with the computers and their communication. In other cases, there may be a deliberate decision to sub-divide some overall system task into activities that can be readily automated, and those that are best left to human beings to carry out. Systems that incorporate human components in either of these ways are termed here computer-based systems. 

Advances in technology can facilitate the generation and transfer of patient documentation. As more pharmacies use the Internet as a means of communication, information can be transferred quickly and accurately over greater distances. Handheld computers and specialty software allow health care practitioners to document information in an electronic format that can be transformed immediately for rapid transfer to others. Reports in the literature have described methods to assess pharmacist interventions related to medication errors, the use of computer-based systems, and recently, the use of personal digital assistants (PDAs) in specific patient care areas. Many of these documentation systems tend to be individualized apphcations in which the transfer of data to other providers is not possible or quite limited. Often these systems focus on the generation of reports for workload analysis or accreditation purposes. 

Thomd, B. (editor), 1993, Principles and Practice of Computer-based Systems Engineering, Wiley series in software based systems... 

From the above description, we may conclude that flowsheeting software is a very sophisticated computer-based system, and not a collection of algorithms for solving different unit operations. A process simulator must be designed with computer science development and management tools. It is interesting to note that in the total cost the software maintenance (typically more than 70 %) is by far more important than the cost of programming (typically under 10 %). 

Some computer-based systems can be partially upgraded, if necessary, through addition of supplemental hardware or by software changes. 

Implementations of I C functions important to safety in nuclear power plants are increasingly realized with computer based systems, i.e. by its software. Often so called equipment families are used to develop these I C functions. Besides a hardware platform, these equipment families provide predeveloped software in the form of basic components from which I C functions can be composed by configuration and parameterization but also larger components which have been developed by conventional software engineering, as e.g. operating systems, I/O drivers, self-supervision software, etc. are included. Outside the equipment families, it may be desirable to introduce also other types of predeveloped software, e g. for simulation and for analysis purposes. 

The assessment and qualification of software for computer based systems important to safety requires (as e g. in lEC 60880) a set of detailed documents according to the development steps of the software life cycle. For pre-developed software the amount of documentation available and its detail will not be sufficient in most cases. On the other hand, the pre-developed software may have been operating in many applications, and it should be possible to evaluate this operating experience to demonstrate dependability. 

IAEA Safety Guide No. NS-G-1.1 Software for computer based systems important to safety in nuclear power plants. Sept. 2000... 

As already mentioned, lEC 60880 describes the modification process for software in computer-based systems of the highest safety category. Taking into account, that most of the software-based systems important to safety are also to be installed in lower safety classes, the importance and necessity of... 

INTERNATIONAL ATOMIC ENERGY AGENCY, Software for Computer Based Systems Important to Safety in Nuclear Power Plants, Safety Standards Series No. NS-G-1.1, IAEA, Vienna (2000). 

Gacek, C., De Lemos, R. Architectural Description of Dependable Software Systems. In Besnard, D., Gacek, C., Jones, C. (eds.) Proceedings of Structure for Dependability Computer-Based Systems from an Interdisciplinary Perspective, pp. 127-142. Springer, Heidelberg (2006)... 

The IAEA has issued a Technical Report [4] to assist Member States in ensuring that computer based systems important to safety in nuclear power plants are safe and properly licensed. The report provides information on current software engineering practices and, together with relevant standards (such as Ref [5]), forms a technical basis for this Safety Guide. 

The guidance is applicable to systems important to safety as defined in Refs [2, 3]. Since at present the reliability of a computer based system cannot be predicted on the sole basis of, or built in by, the design process, it is difficult to define and to agree systematically on aity possible relaxation in the guidance to apply to software for safety related systems. Whenever possible, recommendations which apply only to safety systems and not to safety related systems are explicitly identified. 

The guidance relates primarily to the software nsed in computer based systems important to safety. Guidance on the other aspects of computer based systems, such as those concerned with the design of the computer based system itself and its hardware, is limited to the issues raised by the development, verification and validation of software. 

These advantages are counterbalanced by a number of disadvantages. Software implementation tends to be more complex and therefore more prone to design errors than implementation of purely hard-wired systems. Moreover, software implementations are discrete logic models of the real world. This has two types of consequences. Software is more sensitive (i.e. less tolerant) to small errors. It is also more difficult to test, because interpolation and extrapolation are much more difficult to apply to computer based systems than to traditional analog systems, and ultimately are not entirely valid. 

FIG. 1. Development of software for computer based systems important to safety (numbers refer to sections of this Safety Guide). 

Defence in depth as applied in the design of nuclear power plants [1, 10] should be used in the development of the computer based system and its associated software. If a software based system constitutes the main safety feature, defence in depth, for example by means of a diverse secondary protection system, should be provided. 

The reliability of computer based systems can be enhanced by using diversity to reduce the potential for software common cause failures. The use of diverse functions and system components at different levels of the design should be... 

---